Ari - thank you for taking the time to comment.
Please note the 700,000 new members were for Facebook per day. I made no assertions as to the mainstreaming of Twitter, rather my argument is that Facebook has already gone mainstream. To verify that assertion one needs only to the look at the Facebook growth stats for 2009.
My point regarding Twitter was completely different. That Twitter, while much less mainstream, is becoming the place where news breaks and where buzz builds and then leaks over into other mainstream news media.
The interesting thing about Twitter is not that it is likely to go mainstream, rather that it presages the beginning of real time news. We are already seeing that television news cannot keep up with real time reactions in a disaster in the same way that people on the spot can share updates & report news via Twitter and associated photo or video blogging applications.
Poll people who don't use or understand the medium of Twitter all you like. But incidents like the plane crash in the Hudson photo & the realtime reactions of bystanders who saw that accident & shared their reactions/experiences are the beginning of a reshaping of our news media.
Before this news was broken and shared by professional journalists, now an ordinary person with mobile phone & a Twitter account can do the same thing. That's why many of the journalists I know are actively monitoring Twitter for breaking news now.
Do I think everyone will sign up for & like Twitter? No.
Do I think that the immediacy & ability for bystanders to share their experiences in realtime via Twitter will change things for the rest of the news media? Absolutely!
Don't forget 2 years ago we all thought that Second Life would change the way business worked. And we were wrong. But in the case of realtime media sharing and collaboration the genie is out of the bottle. And this shift is supported by communications devices (like the iPhone or other SmartPhones) that facilitate sharing and collaboration by ordinary people.